How to crop a masked area (polygon shape) out of an image?

Hi all,

Now my code is like below:

I = imread('moon.tif');
figure;
imshow(I)
Poly_Mask = images.roi.Polygon(gca,'Position',[100 150; 200 250; 300 350; 150 450]);

As you can see, the example image is now masked with a polygon shape. I would like to crop the masked area (polygon shape) out of the image. Could you please suggest me how to do this? Thank you.

Answer 1

[Image Analyst]

Try this:

grayImage = imread('moon.tif');
[rows, columns, numberOfColorChannels] = size(grayImage)

rows = 537

columns = 358

numberOfColorChannels = 1

subplot(2, 1, 1);
imshow(grayImage, []);
axis('on', 'image');
title('Original Image')
x = [100, 200, 300, 150];
y = [150, 250, 350, 450];
% tack on starting point to close it so the plot is not oepn jaw. (Only
% needed if you wan to plot the outline in the overlay.
x = [x, x(1)];
y = [y, y(1)];
hold on;
plot(x, y, 'r-', 'LineWidth', 2);
mask = poly2mask(x, y, rows, columns);
% Erase outside mask by setting to zero
grayImage(~mask) = 0;
% Crop
croppedImage = grayImage(min(y):max(y), min(x):max(x));
subplot(2, 1, 2);
imshow(croppedImage)
axis('on', 'image');
title('Cropped Image')

[Chutiphon Moranon]

Thank you so much. I would like to ask for the method to make the black background transparent.

As I replied to Faraz, I would like the image to be transparent for being used with the normxcorr2 function for more accuracy. Will it be possible and how? Thank you.

[Image Analyst]

You can't make it transparent, and it does not need to be for any reason that I can think of, certainly not for normxcorr2().

You can have any shape you want just by putting in the coordinates of the shape boundaries into poly2mask().

Answer 2

[Tala]

I cannot see your image. But I used a photo of my dog (because she wont sue me :D) to show the concept:

I=rgb2gray(imread("Tala1.jpg"));

x=[100 200 300 150];% your ROI x values
y=[150 250 350 450];% your ROI y values 
[rows, columns,~] = size(I);
mask = imcomplement(poly2mask(x, y, rows, columns));
mul = immultiply(I,mask);
imshow(mul)

[Chutiphon Moranon]

Hi Faraz. Love your dog :). Thank you so much for your reply, really appreicate it. I apologize for being ambigious at first.

I have attached an image that explains the desired result below. I would like to crop an area in an image and then bring it out for further use (such as template matching).

For now I use the polygon shape and I cannot crop it by using the rectangular function like 'imrect'. Could you please help suggest me how to get the desired result.

Thank you so much.

[Tala]

do you need to crop a polygon? you could crop out a rectangular section of image with imcrop(I, [x y dx dy]) and then calculate corrolation or convolution for feature matching.

[Chutiphon Moranon]

Thank you for your reply. I will use normxcorr2 as you suggested. However, I would like to use an input image that is not rectangular in shape.

I am trying to crop an input image, only the part area I want for more accurate correlation. Therefore the background area of the cropped image will be transparent as the image below. I am not sure whether this method will provide good result but I will try it. Please feel free to provide your suggestions.Thank you.